WASHINGTON (Reuters) - Sunscreen labels should spell out precisely how well they protect against sun damage and manufacturers should conduct new tests for a subtler type of ultraviolet radiation that can cause cancer, U.S. regulators said on Thursday.

The U.S. Food and Drug Administration proposed testing and rating the level of protection in over-the-counter sunscreens against ultraviolet "A".

Ultraviolet rays from the sun are the leading cause of skin cancer, the most common cancer in the United States. There are two types of UV rays: UVA rays are responsible for tanning, and UVB rays, which lead to burning. The FDA wants to make it clear to consumers that both types are equally dangerous.

Richard Wiles, executive director of the consumer-oriented Environmental Working Group, noted that an FDA proposal in 1999 encountered industry opposition and was never put into effect.

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As it stands, if a product provides minimal UVA protection, makers can claim it, he said, adding the new rule would "change things dramatically from the consumer end."

A new warning in bold type on product labels would remind consumers that using sunscreen should not be a reason for extending time in the sun and that "UV exposure from the sun increases the risk of skin cancer, premature skin aging, and other skin damage."

A new UVA rating system would use a scale of one to four stars, with four representing the highest UVA protection available in an over-the-counter sunscreen.

The FDA said arriving at labeling pertaining to UVA was more complicated and challenging than with UVB because of a lack of international standards and a many testing options.

The public and manufacturers have 90 days to comment on the proposal.

Large sunscreen manufacturers include Johnson & Johnson, Playtex Products Inc and Schering-Plough. All three companies said they were reviewing the FDA proposal.